Mix ratios are by volume, not weight.
Strongest cement & sand mix. Mixes with more cement microcrack on setting, giving lower strength.
4:1 - 7:1
Weaker mixes from 4:1 to 6:1 and sometimes 7:1 are used for walls, drives, bedding brick edging, etc
Strong concrete mix
A lime and cement mix that can sometimes fail prematurely. Not recommended.
A lime & cement mix good for brickwork
Lime mortar for historic brickwork & stone walls. Avoids damage to soft brick & stone. Mixes used are often anything from 2:1 to 3:1.
Mortar mixes are often used dry for bedding fence posts. Rain and water within the soil will gradually set the mix, and the aggregate content keeps the post stable whether its set or not. A dry mix is immediately mechanically stable, whereas a wet mix is more likely to allow the post to move until set.
12:1 is just enough cement to stop sand being washed away under paving. Its a very weak mix of course.
12:1 is or was also used for coal dust: 12 parts coal dust to 1 part cement makes burnable coal lumps.
There are many additives and alternatives that have been used in mortars.
Clay, sand & cement makes a cheaper lower energy mortar. Clay content makes for less ideal handling characteristics.
River silt has been used in place of sand. The result is a strong mortar with less consistent strength than graded sand based mortar.
Coal ash with or without sand, plus lime makes black mortar. Long known for tendency to premature failure. Used to be popular in Northern industrial areas, where large quantities of coal ash needed disposal.
Fibre reinforced cement
Fibres added to sand & cement increase tensile strength and control cracking.
Many fibres can be used, the most popular is now alkali resistant glass fibre. Asbestos fibre used to be the most popular until the mid 80s.
Sand, cement & finely shredded expanded polystyrene make a lightweight insulating mix.
Sand, cement & aluminium powder make a lightweight insulating mix.
No fines concrete
Cement, water & stones makes a highly porous 'no fines' concrete.
- Used to be used for interior construction blocks.
- Occasionally used to facilitate floor drainage.
- Vulnerable to freeze-thaw cycle damage
High Alumina Cement
Subsoil & cement.
Subsoil & lime has also been used, but is not as strong.
Topsoil & cement mix is used for gravel drives. The cement is simply sprinkled and raked in before the gravel is laid. This discourages weed growth and gravel sinking into the mud.
Sand, cement & pozzolan
Any of various Pozzolans may be used. Pozzolans reduce the amount of cement needed in the mix. Some pozzolans, such as PFA, make the concrete corrosive to copper pipe. Upto 30% of the cement can be replaced with a pozzolan. Use of pozzolans reduces pH of the set concrete, and modifies its performance properties in some respects.
Final strength depends on water content, with drier mixes yielding higher set strengths. Semi-dry mixes are widely used industrially, They require force to compact. Paving slabs for example are often made from hydraulically pressed semi-dry mixes.
Where cost is critical and rocks plentiful, mixed rocks can be incorporated into the final product. This can reduce concrete use by 2/3 or more. Suited to large concrete constructions such as house foundations.
Animal hair incorporated into mortar can extend its life by crack control.
Straw has occasionally been incorporated into weak mortar mixes to delay its disintegration.
Shells have occasionally been seen in historic mortar. Why I don't know, perhaps from unsieved beach sand?
Granulated & Powdered Stone
These are incorporated into cement mortar to make reconstituted stone mixes, which make the mix look much more like stone.
Can be incorporated.
Broken glass is sometimes incorporated into concrete to make it ratproof - rats can gradually eat through concrete otherwise. This is primarily used for animal housing, where rats try to get to the food. It was also used for the hard bases of earth walls where rodents can otherwise do considerable damage to the earth walls above.
Gypsum is sometimes used to stabilise subsoil to make cast earth building blocks.
There's more to mortar than what you thought o'.